Umbrella base



FIG. 1 is a perspective view of an umbrella base showing the new design;

FIG. 2 is a front view of the design of FIG. 1;

FIG. 3 is a rear view of the design of FIG. 1;

FIG. 4 is a left side view of the design of FIG. 1;

FIG. 5 is a right side view of the design of FIG. 1; and,

FIG. 6 is a top view of the design of FIG. 1. 

CLAIM The ornamental design for an umbrella base, as shown and described. 